1. Field of the Invention
This invention relates to a high-performance optical projection system for use in patterning microelectronics circuit elements, and more particularly relates to a system which separates and uses each of the multiple spectral peaks of an excimer laser system. An optical dispersion subsystem physically separates the broadband laser output into separate narrowband beams which are directed into separate projection subsystems so that different substrate surfaces can be addressed simultaneously.
2. Background of the Invention
Economical manufacturing of the electronic and opto-electronic components requires the fabrication of a great number of microscopic structures on a single large substrate. These structures can be active devices, such as transistors in a flat panel display (FPD) or integrated circuit (IC). The structures may also be passive patterns, such as interconnecting conductors on a printed circuit board (PCB) or multi-chip module (MCM). The large substrate can be a board, a display panel, a silicon wafer, or roll of flexible substrate material. Both the feature size and the substrate size can vary greatly depending on the application. The actual substrates used can vary from several square inches for small modules to several square feet for PCB's and FPD's. Common to all of the applications is the requirement that the system used to produce them has the required resolution over the entire substrate.
To achieve economical manufacturing, the high cost of production equipment must be offset by high throughput. To increase throughput and yield, substrate handling must be minimized to reduce the overall processing time, and also to avoid damage and contamination. This may be accomplished by processing both surfaces of the substrate simultaneously, with perfect alignment between them so that electrical connections may be made from the top surface pattern to the bottom surface pattern by interconnections called plated via-holes.
There is a great need for the development of patterning equipment for the fabrication of electronic products which combines these major performance attributes: high processing throughput, high resolution, the ability to handle large substrate sizes, and the ability to accurately and economically pattern both sides of a substrate simultaneously. An imaging technology which achieves many of these objectives has been described by K. Jain in U.S. Pat. No. 5,285,236, Large-Area, High-Throughput, High-Resolution Projection Imaging System issued Feb. 08, 1994. The referenced patent discloses a projection imaging system in which an integrated stage assembly for both the mask and substrate is used for the seamless exposure of a large-area substrate with high resolution and at a high-throughput. This technology was extended to include two-sided exposure of substrates in pending patent application of K. Jain, Ser. No. 08/889,307, filed Jul. 7, 1997, Simultaneous, Two-Sided, Projection Lithography System. The patent application by K. Jain uses a beamsplitter to generate two beams for exposing the two sides of a substrate.
In this patent the inventor describes how it is possible to further reduce the costs of a lithography system which is used for two-sided exposure of substrates. The required resolution capability of a projection optical system is one of the key parameters that determines the overall cost of a lithography machine. The theoretical resolution, R, of a projection lens is governed by the equation: ##EQU1## Where .lambda. is the illumination wavelength, N.A. is the numerical aperture of the lens, and k.sub.1 is a processing parameter. The shorter the illumination wavelength, the better the resolution capability of the lens for a given N.A. This has led to the design of projection lenses which are optimized for the ultraviolet (UV) wavelength regime. One of the problems facing equipment manufacturers of microlithography machines is the choice of materials for fabricating projection lenses optimized for the UV regime.
High-resolution lithography lenses which are fabricated from only one optical material, such as fused silica, are generally limited in their optical performance by chromatic aberrations. The chromatic dispersion of the lens material results from changes in the effective index of refraction that depend on the wavelength of light. All excimer lasers which are used as UV sources have a radiation spectrum which is sufficiently broad to degrade the optical performance of the lens as a result of the chromatic dispersion of the lens material.
There are two approaches to limiting the effects of chromatic dispersion in lithography lenses: the first approach is to use additional materials to balance the dispersion; and the second approach is to restrict the range of wavelengths from the illumination source. Both of these approaches are described in more detail.
The first approach is to use more than one optical material in the design and construction of the lens. The additional material is chosen so that its dispersion properties balance the dispersion in the original material and minimize the overall chromatic aberrations. This approach is commonly used in the design and construction of optical systems that operate in the infrared and visible regimes where there are a wide variety of suitable materials from which to choose. This approach is more difficult to implement in the ultraviolet wavelength range since there are very few suitable materials. Materials for UV lithographic applications must have low absorption, and must exhibit high homogeneity. This requires that the materials be very pure and free from defects or inclusions; this eliminates most materials as possible candidates.
The second approach to managing the chromatic aberrations of lithography lenses is to reduce the spectrum of the light source. This often leads to a significant reduction in the output power of the source. In the case of KrF lasers the output power is reduced by an order of magnitude. The reduction of the linewidth is often accomplished by the insertion into the laser cavity of additional optical elements which can suffer damage from the high intra-cavity powers which are present. The line-narrowing mechanisms are also sensitive to small thermal changes so that it becomes necessary to actively monitor and control the wavelength output to keep the peak of the emission spectrum from changing. This adds additional complexity to the system design.
There are some excimer laser sources which have multiple peaks in their radiation spectrum with broad ranges in the relative intensity of the peaks. Xenon Fluoride (XeF) lasers, however, have a radiation spectrum in which the power is equally distributed between two different spectral peaks at 351.0 nm and 353.1 nm. This invention separates the dual peaks from the same XeF laser, generating two radiation beams which are used in separate optical systems to illuminate the top and bottom surfaces of a single substrate. One beam generated from the first spectral peak illuminates a mask in the object field of a projection optical system to expose one side of a two-sided substrate, while the radiation from the second peak is used to illuminate a different mask in the object field of an identical projection optical system to expose the other side of a two-sided substrate. This approach reduces the spectrum seen by each optical projection system to a single narrowband source without sacrificing any of the available power from the laser system.